First of all, OFDM, OFDMA and SC-FDMA schemes used by the present invention are explained as follows.
Recently, the demand for high speed data transmissions rises. And, OFDM, which is suitable for a scheme advantageous for the high speed transmission, has been adopted as a transmission scheme for various high speed communication systems.
In the following description, OFDM (orthogonal frequency division multiplexing) is explained. A basic principle of OFDM lies in dividing a high-rate data stream into a number of slow-rate data streams and transmitting the slow-rate data streams simultaneously using a plurality of carriers. In this case, each of a plurality of the carriers is called a subcarrier. Since orthogonality exists between a plurality of the carriers of OFDM, even if frequency components of the carriers are mutually overlapped, they can be detected by a receiving end. The high-rate data stream is converted to a plurality of slow-rate data streams via a serial-to-parallel converter, a plurality of the parallel-converted data streams are multiplied by subcarriers, respectively, the multiplied data streams are summed up together, and the corresponding sum is then transmitted to the receiving end. So, OFDMA is a multiple access scheme to assign subcarriers to an overall band according to a data rate requested by multiple users in OFDM.
SC-FDMA (single carrier-FDMA) according to a related art is explained as follows.
First of all, the SC-FDMA is called DFT-S-OFDM as well. The related art SC-FDMA is the scheme mainly applicable to uplink. In the SC-FDMA, spreading operation is first applied in a frequency domain by DFT matrix before generating an OFDM signal, the corresponding result is modulated by the conventional OFDM technique, and the modulated result is then transmitted.
To explain the SC-FDMA scheme, several variables are defined. ‘N’ indicates a number of subcarriers carrying OFDM signal, ‘Nb’ indicates a number of subcarriers for a random user, ‘F’ indicates Discrete Fourier Transform matrix, i.e., DFT matrix, ‘s’ indicates a data symbol vector, ‘x’ indicates a data-spread vector in frequency domain, and ‘y’ indicates a transmitted OFDM symbol vector in time domain.
In SC-FDMA, data symbol (s) is spread using DFT matrix before being transmitted. This is represented as Formula 1.x=FNb×Nbs  [Formula 1]
In Formula 1, FNb×Nb is an Nb-sized DFT matrix used to spread data symbol (s). Subcarrier mapping is performed on the spread vector (x) by a predetermined subcarrier assigning scheme. And, a signal to be transmitted to a receiving end is obtained from transforming the corresponding result into a time domain by an IDFT module. A transmission signal to be transmitted to the receiving end is represented as Formula 2.y=FN×N−1x  [Formula 2]
In Formula 2, FN×N−1 is an N-sized DFT matrix used to convert a frequency-domain signal to a time-domain signal. A cyclic prefix is inserted in a signal ‘y’ generated by the above method to be transmitted. And, a method of generating a transmission signal and transmitting the signal to a transmitting end in the above manner is called SC-FDMA. And, it is able to control a size of DFT matrix in various ways for a specific purpose.
The above explanation is based on DFT or IDFT operation. Yet, for convenience of explanation, DFT (Discrete Fourier Transform) or FFT Fast Fourier Transform) operation is used without discrimination in-between in the following description. In case that a number of input values of DFT operation is a power of 2, it is apparent to those skilled in the art that DFT operation can be replaced by FFT operation. So, the content for calling the FFT operation is applicable to the DFT operation as it is.
Generally, in an OFDM system, a plurality of OFDM symbols configure one frame to be transmitted and a preamble is first transmitted for each frame or with an interval of several frames. In this case, the number of OFDM symbols configuring the preamble is generated based on requirements of the OFDM system. For instance, in case of OFDMA based IEEE 802.16 system, a preamble including one OFDM symbol for each downlink frame is first transmitted. And, the preamble is provided to a communication terminal for the purpose of synchronization, cell search, channel estimation and the like in a communication system.
The OFDM system is disadvantageous in that a peak-to-average power ratio (hereinafter abbreviated PAPR) of a transmission signal is considerably high. Since an OFDM signal is transmitted by multi-carriers through an IFFT operation, a size of amplitude of an OFDM signal can be represented as a sum of sizes of the multi-carriers. Yet, if phases of the multi-carriers coincide with each other, the OFDM signal becomes a signal having a high value such as an impulse to have a considerably high PAPR. So, the transmission signal according to the OFDM system lowers efficiency of a high output linear amplifier and operates in a non-linear area of the high output amplifier, thereby causing signal distortion.
FIG. 1 is a structural diagram of a downlink subframe in IEEE 802.16 system.
Referring to FIG. 1, the preamble consisting of one OFDM symbol is transmitted ahead each frame and utilized for time and frequency synchronization, cell search, channel estimation and the like.
FIG. 2 is a diagram of a set of subcarriers carrying a preamble transmitted from 0th sector in IEEE 802.16 system. Both ends of an assigned bandwidth are used as a guard band. If the number of the sectors is three, each of the sectors is transmitted in a manner of inserting a sequence with an interval of three subcarriers and inserting ‘0’ (zero) in the rest of the subcarriers.
A related art sequence used for the preamble is explained as follows. The sequence used for the preamble is shown in Table 1.
TABLE 1ID-IndexcellsectorSequence (hexadecimal)000A6F294537B285E1844677D133E4D53CCB1F182DE00489E53E6B6E77065C7EE7D0ADBEAF110668321CBBE7F462E6C2A07E8BBDA2C7F7946D-5F 69E35AC8ACF7D64AB4A33C467001F3B22201C75D30B2DF72CEC9117A0BD8EAF8E0502461-FC 07456AC906ADE03E9B5AB5E1D3F98C6E............
The sequence is decided by a sector number and cell ID number. Each defined sequence is converted to a binary signal in an ascending order and is then mapped to a subcarrier through BPSK modulation. In other words, a proposed hexadecimal sequence is converted to a binary sequence (Wk) and the Wk is mapped MSB (most significant bit) to LSB (least significant bit). (0 is mapped by +1 and 1 is mapped by −1. For instance, Wk for hexadecimal ‘C12’ in 0th segment having an index 0 is 110000010010 . . . , a converted binary code value becomes −1 −1 +1 +1 +1 +1 +1 −1 +1 +1 −1 +1 . . . .
The above-explained related art sequence is found by a computer simulation to search types of sequences configured with binary codes for a sequence capable of sustaining prescribed correlation properties and a low PAPR (peak-to-average power ratio) in transform to a time domain. If a system configuration is changed or the sequence is applied to a different system, a new sequence should be found.
A newly proposed sequence used for 3GPP (3rd generation partnership project) LTE (long term evolution) technology is explained as follows.
Various sequences are used for the LTE system. And, a sequence used for a channel of LTE is explained as follows.
First of all, user equipment first carries out synchronization with a base station on a synchronization channel (hereinafter abbreviated SCH) and a cell search to communicate with the base station.
A series of processes for obtaining a cell ID, to which the user equipment belongs and establishing the synchronization with the base station is called a cell search. Generally, the cell search is classified into an initial cell search initially performed by a user equipment's ‘power-on’ and a neighbor cell search that a user equipment in idle mode searches neighbor base stations.
The SCH (synchronization channel) is able to have a hierarchical structure. For instance, P-SCH (primary SCH) and S-SCH (secondary SCH) are applicable.
The P-SCH and the S-SCH can be included in a radio frame by various methods. FIG. 3 and FIG. 4 are diagrams to explain various methods of including P-SCH and S-SCH in a radio frame. Under various circumstances, SCH can be configured in LTE system according to the structure shown in FIG. 3 or FIG. 4.
Referring to FIG. 3, P-SCH is included in a last OFDM symbol of a first subframe. And, S-SCH is included in a last OFDM symbol of a second subframe.
Referring to FIG. 4, P-SCH is included in a last OFDM symbol of a first subframe. And, S-SCH is included in a last second OFDM symbol of the first subframe.
The LTE system is able to establish time and frequency synchronizations using P-SCH. And, cell group ID, frame synchronization information, antenna configuration information and the like can be included in S-SCH.
A configuring method of P-SCH proposed by the related art 3GPP LTE system is explained as follows.
First of all, P-SCH is transmitted via 1.08 MHz band centering on carrier frequency. This corresponds to seventy-two subcarriers. In this case, an interval between the subcarriers is 15 kHz. This numerical value is decided because twelve subcarriers are defined as one RB (resource block) in the LTE system. In this case, seventy-two subcarriers correspond to six RBs.
The P-SCH, which is used for a communication system using a plurality of orthogonal subcarriers such as OFDM or SC-FDMA, preferably meets the following conditions. First of all, autocorrelation properties should be good in a time domain for to enhance detecting performances of a receiving end.
Secondly, complexity due to synchronization detection should be low.
Thirdly, N× repetition structure is preferable for excellent frequency offset estimation performance.
Fourthly, PAPR (peak-to-average ratio) or CM should be low.
Fifthly, if it is able to utilize P-SCH for channel estimation, a corresponding frequency response preferably has a constant value. In particular, in aspect of channel estimation, it has been known that a flat response in frequency domain has best channel estimation performance.
However, although the related art has proposed various sequences, it is still incapable of meeting the above mentioned various conditions.